Group transmissions with a multi-gear main transmission and at least one range group drive-connected downstream from the main transmission have been known for a long time and are preferably used in heavy-goods vehicles. Often, a multi-gear splitter group is also drive-connected upstream from the main transmission. By virtue of a usually two-step range group with a gear interval that is above the total gear interval of the main transmission approximately by an average gear interval between two consecutive gears of the main transmission, the spread of the transmission as a whole is approximately doubled and the total number of gears of the group transmission is also doubled. By virtue of a usually two-step splitter group with a gear interval corresponding to approximately half of an average gear interval between two successive gears of the main transmission, the gear intervals of the main transmission are halved and the total number of gears in the group transmission is again doubled. Thus, in combination with a three-gear main transmission with three forward gears and one reverse gear, a group transmission with a total of twelve forward gears and a maximum of four reverse gears is obtained. In combination with a four-gear main transmission comprising four forward gears and a reverse gear, a sixteen-gear group transmission with a total of sixteen forward gears and a maximum of four reverse gears is obtained.
Compared with a single transmission having a comparable number of gears and a similar gear gradation and spread, the group transmission has much more compact dimensions and is lighter. However, since many gear shifts in group transmissions require the changing of gears in more than one partial transmission and therefore take place in a relatively complex manner, shifting processes in most of the known group transmissions are either partially or fully automated.
A typical embodiment of a group transmission with a main transmission, a splitter group connected upstream from the main transmission and a range group connected downstream from the main transmission, is described in DE 44 22 900 A1. In this known group transmission the main transmission is of countershaft design, having two countershafts and one main shaft, between which shiftable spur gear stages are arranged and provide three forward gears and one reverse gear. The splitter group is also of countershaft design and comprises two spur gear stages, commonly known as shiftable input constants, which are arranged between the input shaft of the group transmission and the correspondingly extended countershafts of the main transmission. When the first input constant on the input side is engaged, the spur gear stage of the second input constant on the transmission side serves as a further shiftable spur gear stage of the main transmission. Furthermore, by the direct connection of the input shaft to the main shaft, a direct forward gear can be engaged. The range group is of planetary design and comprises a simple planetary gear set with a sun gear, a planetary carrier carrying a number of planetary gearwheels, and a ring gear, wherein the sun gear is connected in a rotationally fixed manner to the main shaft, the planetary carrier is connected rotationally fixed to the output shaft of the group transmission, and the ring gear can optionally either be securely fixed to the housing in order to engage a gear ratio for a driving range with lower speeds, or connected rotationally fixed to the output shaft in order to engage a gear ratio for a driving range with higher speeds.
Differing from the group transmission known from DE 44 22 900 A1 the splitter group can also be of planetary design and the range group of countershaft design. For example, from EP 0 618 382 B1 various embodiments of a group transmission are known, the transmission comprising a multi-gear main transmission of countershaft design and an auxiliary transmission connected downstream from the main transmission. The auxiliary transmission comprises a two-step splitter group and a two-step range group. In a first embodiment of the auxiliary transmission according to FIG. 2 of that document, the splitter group is of countershaft design whereas the range group is of planetary design, similar to the group transmission known from DE 44 22 900 A1. In a second embodiment of the auxiliary transmission according to FIG. 3 of the document, in contrast the splitter group is of planetary design and the range group is of countershaft design.
Finally, from EP 1 825 168 B1 a group transmission is known, which comprises a main transmission of countershaft design, a two-step splitter group also of countershaft design and connected upstream from the main transmission, and a range group of planetary design and connected downstream from the main transmission. In a manner known per se the range group is in the form of a simple planetary gear set with a sun gear, a planetary carrier carrying a number of planetary gearwheels, and a ring gear, the sun gear being connected in a rotationally fixed manner to the main shaft of the main transmission. Besides the engagement of two gear ratios for driving ranges with lower and higher driving speeds, in this range group a reversing stage can also be engaged for a reverse driving range.
When the driving range for lower driving speeds is engaged, the ring gear is locked onto the housing by a first shifting element and the planetary carrier is connected in a rotationally fixed manner to the output shaft of the group transmission by a second shifting element. When the reversing stage for the reverse driving range is engaged, the planetary carrier is locked onto the housing by the first shifting element and the ring gear is connected in a rotationally fixed manner to the output shaft by means of the second shifting element. When the driving range for high driving speeds is engaged, the first shifting element is in an open, neutral condition (with no shifting function) and the ring gear and the planetary carrier are connected in a rotationally fixed manner, via the second shifting element, to one another and to the output shaft.
By virtue of the shiftable reversing stage in the range group, advantageously there is no need to arrange a shiftable reversing stage in the main transmission; in other words the reversing stage present therein can be omitted or replaced by a spur gear stage for an additional forward gear. To carry out the shifting function of the range group, however, two shifting elements are needed, the first of which is in the form of a dual shifting element with two shift positions and a neutral position while the second shifting element is a triple shifting element with three shift positions, including a shift position with triple coupling. Thus, the range group design known from EP 1 825 168 B1 has the disadvantage that its structure is complex, in particular that of the second shifting element, and that it requires two shift actuators for the simultaneous actuation of the two shifting elements.
A group transmission of similar type having a range group comprising two planetary gear sets arranged coaxially with one another and radially one over the other is known from the previously unpublished DE 10 2013 202 045 A1. It is not known from that document how the two planetary gear sets are centered and mounted radially and axially relative to one another.